<scp>COVID</scp>‐19 <scp>mRNA</scp> vaccine‐associated cerebral venous thrombosis: Rare adverse event or coincidence?
Authors: Bingwen Eugene Fan, Ryan Ruiyang Ling, Kollengode Ramanathan, Bernard Pui Lam Leung, Xin Rong Lim, Veerendra M. Chadachan, Umapathi Thirugnanam, David Stegner, Tian Ming Tu
Journal: American Journal of Hematology (2022)

Rare cases of cerebral venous thrombosis after COVID-19 mRNA vaccines have been reported in real-world post-vaccination data. As phase 3 clinical mRNA vaccine trials do not aim to and are not sufficiently powered to detect rare adverse events, the risks, and benefits of new vaccines are usually not fully known during regulatory approval, particularly following emergency use authorization. National regulatory bodies such as the Food and Drug Administration (FDA) and European Medicines Agency (EMA), and pharmacovigilance databases such as the World Health Organization's (WHO) platform VigiBase, the United States' Vaccine Adverse Event Reporting System (VAERS) and EMA's EudraVigilance are instrumental in ensuring vaccine safety and shaping public health policy. A temporal relation to vaccine administration does not confirm the diagnosis of "vaccine-induced thrombosis." In addition, available literature on post-vaccination thrombosis lacks sufficient power to detect significant associations with rare events like CVT. Hence, we discuss our observations of cerebral venous thrombosis occurring after COVID-19 mRNA vaccination in Singapore. Mass COVID-19 vaccination in Singapore commenced on December 30, 2020 with the Pfizer-BioNTech/Comirnaty BNT162b2 vaccine, while Moderna mRNA-1273 vaccination was initiated on March 12, 2021. We initially reported three cases of CVT in three patients (median age 60) within 10 days of receiving a second dose of mRNA-COVID-19 vaccination, of which two required decompressive craniectomy.1 Platelet count was normal in these three patients, with two patients further testing negative for heparin-induced platelet aggregation test (HIPA). We performed platelet flow cytometry (Becton Dickinson FACSCanto TM Flow analyzer) on two of these three patients with CVT; both demonstrated increased surface expression of CD62P (platelet P-selectin) and PAC1 (activated GP IIb/IIIa) in response to an ADP agonist. Subsequently, we performed a retrospective cohort study comparing the incidence and clinical characteristics of CVT following SARS-CoV-2 infection or receipt of at least one dose of mRNA-COVID-19 vaccination from January 23, 2020, to August 3, 2021.2 Among 3 006 662 individuals who received at least one dose of mRNA-COVID-19 vaccines, 1 626 623 (54.1%) were male and the median age was 50 years. Nine CVT cases were identified (seven males; median age: 60 [46–76] years). Six patients received the BNT162b2 vaccine, and three patients received the mRNA-1273 vaccine. None of the nine patients suffered from fatal CVT. They presented (primarily with focal neurological deficits and headaches) 28 (19–39, median IQR) days after the first dose, and 7 (1–11, median IQR) days after the second. Platelet count was normal, and four patients had mildly reduced antithrombin III [n = 2] and protein C [n = 1] levels possibly due to consumption of these natural anticoagulants during acute thrombosis, while two patients had a lupus anticoagulant. One patient was weakly positive for PF4 antibody ELISA, with optical density <1 (Asserachrom® HPIA – IgG, Diagnostica Stago; threshold to positivity <0.2). Three patients tested negative for HIPA. The incidence of CVT hospitalizations was 2.93 per 100 000 person-years (95% CI, 1.18–6.04) after receiving two doses of mRNA-COVID-19 vaccine and 2.59 per 100 000 person-years (95% CI, 1.19–4.92) after the first dose of mRNA-COVID-19 vaccine. Recently, the Health Sciences Authority (HSA), a WHO maturity level 4 national regulator of health products in Singapore, reported a small increase in incidence of CVT post-mRNA vaccination.3 As part of safety updates on COVID-19 vaccination, HSA reported 13 suspected cases of CVT following more than 12.7 million doses of mRNA-COVID-19 vaccination. This data were derived from comparison of CVT incidence during the pre-pandemic period (2019) to CVT incidence during the COVID-19 pandemic from a national database derived from hospitalized cases (ICD10 discharge diagnosis codes of CVT). The key update states that "Very rare cases of CVT have been reported with the [mRNA-] COVID-19 vaccines, both overseas and locally. CVT is a very rare type of blood clot […] which can happen naturally regardless of […] vaccinat[ion]. The incidence of CVT has stabilized and remains very rare. Although there is a small increase in incidence of CVT with mRNA vaccines (about 1 additional case per million doses), the risk of CVT after COVID-19 infection is much higher after mRNA vaccination (about 30 cases per million infections), and the benefits of vaccination continue to outweigh the small increased risk of CVT." Several observational studies evaluating the risk of CVT show discordant findings post mRNA-COVID-19 vaccination but showed a clear signal for modified adenovirus vector vaccines (Table 1). In a nationwide cohort study, Andrews et al.6 showed no increased incidence of CVT for BNT162b2 vaccine recipients of any age. However, based on the EMA's Eudravigilance database, Krzywicka et al.5 did not find an increase in the absolute risk of CVT following BNT162b2 (0.6 per million doses, 95% CI 0.5–0.7), or mRNA-1273 vaccination (0.6 per million doses, 95% CI 0.3–1.1). In contrast, a study by Abbattista et al.4 showed an increased reporting rate of CVT per million person vaccinated-days following BNT162b2 vaccination (1.92, 95% CI 1.71–2.12) and mRNA-1273 vaccination (5.63, 95% CI 4.74–6.64) for CX-024414 (mRNA-1273). These findings concur with those presented by Park et al.,7 who in their analysis of WHO's Vigibase, identified 756 (0.07%, 620 [0.05%] after BNT162b2 and 136 [0.01%] after mRNA-1273) cases of CVT out of 1 154 023 mRNA-COVID-19 vaccine-related ADRs within 28 days of vaccination. In their disproportionality analysis, they found a potential safety signal for COVID-19 mRNA vaccines (95% lower end of information component = 1.56; reporting odds ratio with 95% CI: 3.27). Nonetheless, while pharmacovigilance databases contain large datasets, they are limited by underreporting, reporting biases, and diagnostic inaccuracies. Two interesting epidemiological observations stand out from the observational studies mentioned above. First, 80% of our patients with CVT were male, which stands in contrast with the epidemiology of vaccine-induced thrombotic thrombocytopenia (VITT), which predominantly affects females. In their analysis, Park et al.7 reported on 294 males (39.1%) with mRNA-vaccine associated CVT. This differs from the classic epidemiological presentation of CVT which is more common in females than males (3:1). Second, in our cohort study, CVT following mRNA-vaccination presented 28 days after the first dose, and 7 days after the second dose. In contrast, the onset of CVT after adenoviral vector-based vaccines was shorter (7–10 days after the first dose); however, with similar observations that patients after the second dose of ChAdOx1 nCoV-19 vaccine also presented earlier than after the first dose.8 Symptom onset of VITT may be more rapid after a second than after a first dose. Park et al. found that the median time to onset (days) of CVT in BNT162b2 vaccines (median 12 days, IQR 3–22) was significantly shorter than ChAdOx1 nCoV-19 vaccines (11 days, IQR 7–16, p = 0.02). These findings suggest that the mechanisms behind mRNA-COVID-19 vaccine-associated CVT differ from VITT-associated CVT in adenoviral vector COVID-19 vaccines, and CVT that is not associated with vaccination. The exact mechanism of mRNA-vaccine-associated CVT remains unclear. Prior studies did not find hypercoagulability or platelet aggregation in healthy volunteers following mRNA COVID-19 vaccination, or differences in hemostatic and endothelial activation before and after mRNA COVID-19 vaccination.9, 10 Therefore, mechanisms involving circulating soluble spike antigen causing cerebral venous sinus endothelial activation and the loss of blood–brain barrier integrity post-vaccination are less probable. While circulating spike glycoproteins have been found using an ultra-sensitive SIMOA assay,11 the mean plasma spike glycoprotein peak level was 62 ± 13 pg/ml (or 0.344 nanomoles [nM]), markedly lower than the SARS-CoV-2 dissociation constant of 14.7 nM,12 that is, the pathological levels required for binding to angiotensin-converting enzyme 2 receptors on endothelial cells. Given the persistent increase in the expression of platelet activation markers CD62P and PAC1 (activated GP IIb/IIIa) in flow cytometry 4 weeks post-decompressive craniectomy in one of our patients,1 we postulate that a possible cause of post-mRNA-vaccine CVT could be a rare immune-mediated activation of platelets, and the increased upregulation of platelet GPIIb/IIIa, however, results in a single patient may not be sufficient; hence, these findings need to be substantiated with more studies. Additionally, correlation with novel mechanisms such as antibody-induced cooperative signaling of two platelet receptors, C-type lectin-like receptor-2 (CLEC-2) and GPIIb/IIIa (which resulted in severe, isolated CVT with no thrombosis in other sites in murine models)13 are required. Vaccines remain the most effective means to prevent serious COVID-19 disease, and their benefits continue to outweigh their risks. While mRNA-COVID-19 vaccination-associated CVT remains an extremely rare, but serious adverse event, further well-designed epidemiological studies of existing pharmacovigilance databases and evaluation into the mechanism of action are much needed. Conceptualization: Bingwen Eugene Fan and Tian Ming Tu. Drafting of the manuscript: Bingwen Eugene Fan. Acquisition, analysis or interpretation of data and critical revision of manuscript for important intellectual content: All authors. The authors greatly appreciate the efforts of our fellow healthcare workers during this pandemic. The authors wish to especially acknowledge Felicia Liying Tan, Jonathan Jia En Boey, Christopher Jer Wei Low, and Yeung Jek Ho for their assistance with the data collection for Table 1. The authors declare no conflict of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request. The data that support the findings of this study are available from the corresponding author upon reasonable request.